The present invention relates to an internally cooled cable for transmitting high electrical energies, and particularly to a high-voltage, high-energy cable comprising an electric conductor surrounding a channel for conducting a cooling medium, an outer tubular member surrounding the electric conductor, a cable jacket surrounding the outer tubular member and insulation intermediate the outer tubular member and the cable jacket and separating one from the other.
Internally cooled, particularly water cooled, high-energy cables are already known, for instance from the German published patent application No. 1,960,546 or from the publication Draht Coburg 21 (1970) No. 4, entitled "Wassergekuehltes Kabel fuer die CERN" (Water-cooled Cable for the CERN), pages 230 to 233. However, the cables disclosed in these publications are not suited for transmission of high electric energies in the region up to 2000 MVA. This can be easily established when the selected cable dimensions, such as the diameter of the electric conductor or the diameter of the cooling channel, are considered.
Furthermore, it is known, for instance from the U.S. Pat. No. 3,509,266, to provide a high-voltage cable with a high energy-transmitting capacity, which includes an electric conductor which is sealed and which is provided with an internal channel through which a cooling medium, such as water, is conducted. However, the electric conductor of this cable is not circumferentially complete but is rather provided with a fluid-impermeable layer on its outer surface in order to prevent the cooling medium from penetrating into the electric insulation. This patent also discloses that the transmission capacity of a cable can be substantially increased by internally cooling the same; however, no information is provided about the dimensioning of such water-cooled high-voltage cable and particularly about the dimensions of the electric conductor so as to minimize the expenses connected with such a cable, which expenses include the expenses of manufacturing and installing the cable and the operating expenses of the cable.
There is also already known, from a commonly assigned copending U.S. patent application Ser. No. 457,778, filed Apr. 4, 1974 to provide a high-energy, high-voltage cable comprising an inner tubular member defining a cooling channel for a cooling medium, the inner tubular member itself serving as, or being surrounded by, an electric conductor, an outer jacket surrounding the electric conductor, and an electric insulation intermediate the electric conductor and the cable jacket and electrically separating the former from the latter. It has been also disclosed in this application that best results are obtained when, in the case that the electric conductor is made of aluminum, the diameter of the channel provided in the inner tubular member is at least 60 mm, preferably equal to or exceeding 70 mm and, in the event that the electric conductor is made of copper, the inner diameter of the channel is at least 70 mm, preferably equal to or exceeding 80 mm. When such dimensions of the channel are selected, it is assured that the expenses connected with the production and the maintenance of the cable are minimized. In other words, the overall expenses, which are obtained by adding the cost of installing the cable to the cost of maintaining the cable and to the cost of electric losses occurring during the operation of the cable are at their optimum, while any deviation from the above-mentioned values of the dimensions of the cable results either in a substantial increase in the cost of production and installation of the cable with a small reduction in the amount of losses or in a significant increase in the amount of losses and the cost of maintaining the cable with a disproportionately small decrease in the production and installation costs so that, when these costs are added, the result is a higher overall expense than that obtained when the dimensions are such as proposed by the present invention. The above-mentioned expenses may be even further reduced with a simultaneous increase in the transmitted electric energy when the thickness of the wall of the electric conductor amounts to substantially 15 mm when aluminum is used for the electric conductor, resulting in a conducting cross-section of at least 3200 mm.sup.2 while, when copper is used for the electric conductor, the advantageous results are obtained when the wall thickness of the electric conductor amounts to substantially 12.5 mm and the conductive cross-section of the electric conductor amounts to at least 3000 mm.sup.2.
Experience has shown that such a cable works satisfactorily under ordinary circumstances; however, it has also been established that there exists the danger that the inner tubular member which defines the cooling channel may develop cracks or fissures permitting penetration of the cooling medium through the inner tubular member and through the electric conductor into the electric insulation, or that the fluid-tightness of the cable may deteriorate in any other way, so that the cooling medium will be permitted to reach and enter into the electric insulation. Once the cooling medium enters the electric insulation, the result is not just a temporary disruption of the function of the cable which would require only the location of the break or fissure and repair of the affected portion after which the cable could be put back into operation, but rather the electric insulation, which may consist of several layers of oilpaper or similar insulating material, suffers permanent damage which renders the entire cable, or a section thereof, provided that the various sections are sealed with respect to one another, unusable.